Amplifier



AMPLIFIER Filed NOV. 29, 1941 INVENTOR R. W. CLARK ATTORNEY ,of voltage such as that Patented July 13, 1943 AMPLIFIER Robert W. Clark, Teaneck, N. 1., assil or to Radio Gorporation'of America, a corporation of Delaware Application November 29, 1941, Serial No. 420,961 7 Claims. (01. 119-171) is desirable to amplify concurrently direct cur-' rent components as well as alternating current components of an exceedingly wide band of frequencies. The necessity for amplifying the direct current components requires a large number of separate power supplies or a singl very high voltage supply, in known systems, each power supply being held constant within very narrow limits by suitable regulators. The requirements of extra separate power supplies and regulating equipment have militated against the use of direct current amplifiers, with the consequence that auxiliary circuits have had to be provided to reinsert components representative of the direct current components in an alternating current amplifier, which circuits are a compromise.

By my invention I overcome the necessity of providing a plurality of separat power supplies for a direct current amplifier, while, at the same time, I provide an amplifier whose response is substantially uniform for both direct current and alternating current components, even though the alternating current components may have frequencies extending to five megacycles per second For the power supply, instead of using a large number of supplies or a single high voltage supply, this system will operateon one supply normally used for audio amplifiers.

Accordingly, it is one of the objects of my invention to provide a new, useful and improved thermionic amplifier capable of amplifying both directcurrent and alternating current components over a wide band of frequencies.

Another object of my invention is to provide a direct current amplifier having a plurality of stages and requiring only a single power supply.

Another object of my invention is to' provide a. direct current amplifier capable of amplifying alternating currents over a wide band of frequencies and at the same time providing means for obtaining a very low impedance output circuit, while utilizing only a single power supply.

A still further object of my invention is to provide a direct current amplifier having but a single power supply and providing uniform gain for both direct current and alternating current components over a wide band, while remaining stable and relatively uninfiuenced by fluctuation in the power supply.

Other objects of my invention will become apparent upon a. reading of the following detailed description of my invention taken together with the drawing.

Figure 1 shows schematically a circuit diagram of a multi-stage amplifier embodying the principles' of my invention, while Figure 2 shows a circuit diagram embodying a modification of the circuit arrangement shown in Figure 1, in which the modification includes means for driving an output from a low impedance circuit.

Turning now to Figure 1, I have shown by way of example a three-stage amplifier. It-will be apparent from what follows, however, that the number of stages may be expanded or contracted in accordance with the requirements for a given use. The signal to be amplified, which may include direct current components as well as alternating current components of a wide band of frequencies, is applied to input terminals I, across which terminals is connected a resistor 3. One side of the resistor is grounded, while the other side is connected to the control electrode of tube 5. The cathode of the tube 5 is connected through resistor to ground, while the anode of tube 5 is connected to one end of the resistor 9. The other end of the resistor 9 is connected to resistor H, which, in turn, is connected to the positive pole of the battery 39. The negative side of the battery is grounded. A condenser I3 is connected to the common junction point of the resistor 9 and resistor II and the other side of the condenser is grounded. Connected between the anode of the tube 5 and the grid of thetube 2| is a parallelly connected resistor l1 and condenser l5. From the junction point of the resistor I! and the grid of the tube 2| is connected a resistor IS, the other terminal of which is grounded.

The cathode of tube 2| is connected through the grounded cathode resistor 22, while the anode of the tube 2| is connected to the serially connected resistors 23 and 25. The other terminal of the resistor 25 is connected to the plus terminal of the battery 39. A condenser 2| is connected to the junction point of the resistors 23 and 25. The tube 33 has its grid coupled to the anode of the tube 2! through the parallelly connected resistor SI. and condenser 29, and the grid of tube 32 is connected to ground through the resistor 22. The load circuit 35 is connected between the anode of tube 33 and the positive terminal of the battery 39. The cathode of tube 3! is connected to ground through the resistor 31.

In operation, the signals impressed on the grid of tube 5 willbe amplified by the tube in accordanc with well known theory of operation of thermionic amplifiers. The resistor I is so chosen in value as to provide appropriate bias to the tube 5 so that the tube operates on the linear portion of its grid voltage-plate current characteristic.

The amplified voltage which appears across the serially connected resistors 9 and I is then transferred to the grid circuit of tube 2| for further amplification and the amplified output of the tube 2| is further amplified by the tube 33 and sent to the load circuit 35. In order that the tubes 2| and 23 may operate on the linear portion of their characteristics, it is necessary that the potential drop between the grid and ,the oathode, in general, be negative and of a value which falls midway on the linear portion of the grid voltage-plate current characteristic of the tubes 2| and 23. These conditions can be met byproper choice of resistances l9 and 20 since En is equal to E minus InRaz; where E21 i equal to the potential difference between the grid and cathode of tube 2|, Em is the potential drop across the resistor I9; I21 is the plate current of the tube 2|, while R22 is the resistance of the resistor 22.

It will be noted that the potential at the junction point of the grid of tube 2| and the resistor I9 is positive with respect to ground, while the cathode of tube 2| is also positive with respect to ground. By insuring that the. potential drop across the resistor 22 is greater than the potential drop across the resistor l9, the grid of tube 2| will be negative with respect to the cathode of tube 2|. Similar consideration holds for the tube 33/ In order that the amplification for the direct current, low frequency alternatin current, and

- the high frequency alternating current components shall be constant, it is necessary that the following relationship be maintained for the tubes 5 and 2|:

where the subscripts refer to the elements shown in Figure 1. Under these conditions, uniform amplification of the entire band from zero to very high frequencies will be obtained, and, at the same time, the phase angle will remain substantially linear as a function of frequency so that completely uniform amplification over the entire band, including the direct current components, is obtained. Similar ratios for the stage following the tube 2| are necessary and become:

while, in the case of tube ,is:

Because of the selected-ratios, it will be readily apparent that the actual voltage transfer beof the resistor 9 is transferred to the grid of the tube 2| so that uniform amplification results.

It will thus be apparent that only a single voltage supply is necessary for providing uniform amplification of a multi-stage amplifier of both the direct current and alternating current components. Moreover, small fluctuation in the supply voltage of the battery 39 will not introduce disturbances, since the voltage rise in the plate circuit is counterbalanced by the change of voltage in the grid circuit, due to the change in potential across the cathode resistors, so that stable operation is aiforded.

In Figure 2, I have shown a modification of the embodiment of my invention shown in Figure 1, whereby with a relatively small change in the circuit arrangement I make it possible to obtain a low impedance output circuit, which impedance output circuit may be made independent of any requirements in the amplifier. For purposes of illustration, in Figure 2 I have shown but two stages, one stage being an amplifier stage similar to the stages shown in Figure 1, while the output stage comprising the tube 18 includes the modification of a low impedance output circuit. In Figure 2 the signals to be amplified, containing both direct current and alternating current components, are impressed on the terminals 5| across the grid cathode circuit of the tube 55. A high impedance 5! serves to prevent the grid cathode circuit from being opened in the event that the source of signals is disconnected from the terminals 5|. A cathode resistor I1 is connected between ground and the cathode of tube it. The plate circuit of tube 55 comprises the serially connected resistors 59 and I with a decoupling capacity 63 connected eifectively across the filter resistor 8|. Coupling to the tube 19 is provided by way of the parallelly connected resistor and capacitor 61 and 65, respectively, and a grid leak resistor TI, connected between the grid of the tube 19 and ground, is provided in order to supply the amplified potentials from the tube II to the grid cathode circuit of the tube 19. A resistor I connected between the junction point of the parallelly connected capacitor and resistor 81 and the grid of the tube 1! to the single power supply 'Il serves to provide proper bias to the grid of the tube II by' means of the variable tap II 01' the potentiometer II. It will be noted that by this connection the cathode resistor 8| connected between the oathode of the tube 19 and ground may be any value desired, without upsetting the grid bias of tube 19, since the grid bias may be set independently by the potentiometer tap II. The plate of the tube 19 is supplied with power through the resistor II, and a decoupling condenser OI serves to maintain the potential of the plate of the tube 1! substantially constant.

Output is obtained across the cathode resistor 8| bymeans of terminals I1. In view of the fact that the cathode resistor 8| may have any value, a low value may be selected in order to feed, for example, a coaxial cable whose impedance may be of the order of 75 ohms. By making the cathode resistor ll seventy-five ohms, perfect termination match is aii'orded so that reflections in the cable are avoided.

In the circuit shown in Figure 2, in order that the amplification remain constant, it is necessary that the following relationships exist:

must equal Rsi/Rer, which, in turn, must equal C63/C65, where the Rs and the Us representing the elements in Figure 2 correspond to the sub scripts. Ordinarily, the resistors 69 and 61 may be suitably chosen to provide the proper grid bias to the grid of the tube'l9, cooperating with the setting of the potentiometer l3.

It'will be appreciated that where extremely high frequencies of the order of five megacycles are desired to be amplified linearly, series "peaking coils may be inserted to overcome stray capacities in series with the resistorst and 23, in Figure l, and in series with the resistor 59 in Figure 2. Such peaking coil circuits are, of course, well known in the art, and, accordingly, are not illustrated here. The patent to Preisman No. 2,243,121, issued on May 27, 1941, for example, shows the .use of peaking coils. It will be understood that for high frequencies the plate circuits may employ shunt peaking circuits or any 'other of the well-known circuits which afford constant impedance over wide bands of frequencies.

Various alterations and modiflcationsof the present invention may become apparent to those skilled in the art and it is to be noted particularly that while triodes have been shown in the drawing merely by way of example, that tetrodes, pentodes, or any other type of thermionic amplitying tubes may be used, and in practice the pentode is ordinarily preferred, since it provides a constant current generator of signals, which is particularly suited for use in my invention. Thus any and all such modifications and alterations are to be considered within the purview ofthe present invention except as limited by the hereinaiter appended claims.

Having now described my invention, what I claim is:

1. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode, a control electrode and an anode, a single source of direct current potential, two serially connected resistors connected between the anode of one of said tubes and the positive terminal of said source, a condenser connected between the junction point of said serially connected resistors and a common point of potential with respect to said positive terminal, a parallelly connected resistor and condenser coupling the said anode to the grid of another of said plurality oi. tubes, means including a resistor connected between the negative terminal of said source of potential and said grid to maintain proper bias on said grid, a resistor connected between said common point and the cathode of said other tube, an anode circuit connected between said positive terminal and the anode of said other tube and means to supply signals to the gridcathode circuit of said first-named tube.

2. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode, a control electrode and an anode, a single source or direct current potential, two serially connected resistors connected between condenser coupling the said anode to the grid of another of. said plurality of tubes, a resistor'connected between the negative terminal of said source of potential and said grid, a resistor connected between said negative terminal and the iacathode of said other tube, an anode circuit connected between said positive terminal and the anode of said other tube and means to supply signals to the grid cathode circuit of said firstnamed tube.

3. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode, a control electrode and an anode, a single source of direct current potential, two serially connected resistors connected between the anode of one of said tubes and the positive terminal of said-source, a condenser connected between the junction point oi said serially connected resistors and a common point of potential with respect to said positive terminal, a parallelly connected resistor and condenser coupling the said anode to the grid of another of said plurality of tubes, a resistor connected between said common point and said grid, a resistor connected between the negative terminal of-said source of potential and said grid; a resistor connected between said common point and the cathode of said other tube, an anode circuit connected between said positive terminal and the anode of said other tube and means to supply signals to the grid cathode circuit of said first-named tube.

4. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode, a control electrode and an anode, a single source of direct current potential, two serially connected resistors connected between the anode oi one of said tubes and the positive terminal of said source, a condenser connected between the junction point or said serially-connected resistors and a common point of potential with respect to said positive terminal, a parallelly the anode of one of said tubes and the positive connected resistor and condenser coupling the said anode to the grid of another of said plurality of tubes, a resistor connected between said common point and said grid, a resistor connected between the negative terminal of said source of potential and said grid, a reslstor'connected between said common point and the cathode of said other tube, an anode circuit connected between said positive terminal and the anode of said other tube and means to derive output energy from across said resistor connected to said cathode.

5. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode, a control electrode and an anode, a single source of direct current potential, two serially connected resistors connected between the anode of one oi. said tubes and the positive terminal of said source, acondenser connected between the junction point of said serially connected resistors and the negative terminal of said source, a parallelly connected resistor and condenser coupling the said anode to the grid of another of said plurality of tubes, a resistor connected between the negative terminal of said source of potential and said grid, a resistor connected between said negative terminal and the cathode of said other tube, an anode circuit connected between said positive terminal and the anode 01 said other tube and means to supply signals to the grid cathode circuit of said firstnamed tube, the ratio of said grid-connected resister to said anode-connected resistor being equal to the ratio of said junction-point connected condenser to said parallel-connected condenser andequalto theratio otsaidcoupllngrefiner to said positive-terminal connected resis- 6. An amplifier comprising a plurality of thermionic tubes, each tube having at least a cathode. a control electrode and an anode, a single source of direct current potential, two serially connected resistors connected between the anode of one of said tubes and the positive terminal oi said source. a condenser connected between the Junction point of said serially connected resistors and a common point of potential with respect to said positive terminal, a parallelly connected resistor and condenser coupling the said anode to the grid of another oi said plurality of tubes, a resistor connected between said common point and said grid, a resistor connected between the negative terminal or said source of potential and said grid, a resistor connected between said common point and the cathode 01' said other tube, an anode circuit connected between said positive terminal and the anode'of said other tube, means to supply signals to the grid cathode circuit oi said first-named tube, and .a load circuit connected across said resistor connected to said cathode, the ratio or capacity of said first-named condenser-to the capacity of said second-named condenser being equal to both the ratio of said coupling resistor to said positiveterminal connected resistor and the ratio or the efi'ective resistance of the two gi'id connected resistors in parallel to said anode-connected resistor.

'1. An amplifier comprising a first and second electron discharge tube each having at least a cathode, a control electrode and an anode, a single source of direct current otential, means-to apply potential variations between the control electrode and cathode of said first tube, a first and second resistance connected in series between the positive terminal of the source of potential and the anode of said first tube, a coupling resistance connected between the anode of said first tube and the control electrode of said second tube, a grid resistance connected between the control electrode of said second tube an the negative terminal Of the source of potential, a first condenser connected in parallel with said coupling resistance, a second condenser connected between the junction of the first and second resistances and the negative terminal of the source oi potential, biasing means connected between the negative terminal of the source oi! potential and the cathode of said second tube, means including said source of potential for maintaining the anode of said second tube positive with respect to its associated cathode. the

values of the resistances and condensers being so chosen that the ratio of the grid resistance to the tending from zero to at least five megacycles per, second can be amplified substantially linearly and without phase shift.

' ROBERT W. CLARK. 

